JPH0329373A - Amorphous solar cell - Google Patents

Abstract

PURPOSE:To improve photoelectric conversion efficiency by forming a window side impurities layer in a multilayer structure of an amorphous silicon layer (fine crystalline layer) including a fine crystal and an amorphous silicon nitride layer (hetero layer) and by specifying the film thickness of the window side impurities layer. CONSTITUTION:A substrate 1 which is equipped with a transparent electrode 2 in texture structure is adopted, a window side impurities layer is in multilayer structure of an amorphous silicon layer (fine crystalline layer) 4 including a fine crystal and an amorphous silicon nitride phase (hetero layer) 3, and the window side impurities layer is formed in film thickness exceeding 100Angstrom , thus reducing light absorption by the window side impurities layer, achieving a light confinement effect with the texture structure at the transparent electrode 2, and achieve a conversion efficiency without reducing opening voltage and fill factor.

Description

DETAILED DESCRIPTION OF THE INVENTION <<Industrial Application Field>> The present invention relates to solar cells using non-quality semiconductors. More specifically, the present invention relates to an amorphous semiconductor solar cell with improved photoelectric conversion efficiency. [Prior Art] In recent years, solar cells using non-quality semiconductors have attracted attention because they are cheaper to manufacture than single-crystalline solar cells, and are being actively developed as a promising solar cell. Various structures have already been proposed for this solar cell, but among these, the intrinsic layer (iJi
) sandwiched between impurity layers (p or n layer) or n-i-p type (hereinafter abbreviated as pin type in this specification) realizes high efficiency. Are known. Conventionally, when these pin-type amorphous silicon semiconductors function as solar cells, the pH or n-layer amorphous silicon absorbs light and becomes a carrier (
It is known that photobiochemical carriers, which serve as photocurrent, are generated in the i-layer because the lifetime of photobiochemical carriers (hereinafter referred to as photobiochemical carriers) is short. However, in the case of pin-type solar cells, the i-layer is sandwiched between the p-layer and the n-layer, so
Light enters the i-layer through the p-layer or n-layer. For this reason,
The p-layer or n-layer is preferably a material with a wide optical gap and low light absorption, but conventional n-type and p-type amorphous silicon have a light absorption coefficient that is similar to that of the amorphous silicon of the i-layer. Since it was about the same level or larger, light absorption loss in the n-layer or p-layer was one of the causes of lowering the photoelectric conversion efficiency. Further, a pin type amorphous silicon solar cell has a structure such as glass/transparent electrode/(pin or nip)/metal electrode, transparent electrode/(pin or nip)/metal, and light is incident from the transparent electrode. Therefore, light loss due to light reflection from solar cells is also light! This prevents the conversion efficiency from increasing. In this case, although it is desirable to control the refractive index of the p-layer and n-layer to minimize light reflection at the interface between the transparent electrode and the p-layer or n-layer, conventional p-layer or nJi For example, the refractive index at 1500 nm is 3.4 to 3.6, which is the same as that of the non-quality silicon of the L layer, and there is a large difference from the refractive index of the transparent electrode, which is approximately 2.0, resulting in reflection loss. Therefore, in order to improve the above-mentioned drawbacks, it is necessary to provide textured structures to transparent electrodes and substrates, and to use amorphous hydrogenated silicon containing microcrystals (μc-Si) as a material for the p-layer or n-layer.
:H) is proposed to be used. However, when μ(-S i :H is used, there is a drawback that the compatibility with the transparent electrode is not sufficient. The inventors of the present invention have conducted extensive research based on the prior knowledge and found that By making the window-side impurity layer a multilayer structure consisting of a non-quality silicon layer containing microcrystals (microcrystalline layer) and an amorphous silicon layer containing certain elements (heterolayer), it is possible to improve the performance of amorphous semiconductor solar cells. We discovered that the performance can be improved and have already proposed it (Japanese Unexamined Patent Publication No. 60-207319). However, even if the invention related to stacked solar cells is simply applied, the open circuit voltage and fill factor will drop significantly, and the desired result will not be achieved. It has been difficult to achieve the conversion efficiency of By adopting this method and increasing the thickness of the impurity layer on the window side to a thickness exceeding 100%, we succeeded in obtaining a solar cell with high conversion efficiency without reducing the open-circuit voltage or fill factor. Therefore, the first object of the present invention is to provide a pin-type non-quality solar cell with high conversion efficiency. A second object of the present invention is to provide an impurity layer with a wide optical gap and low optical loss, which is effective as a window-side impurity layer of a pin-type amorphous solar cell. A third object of the present invention is to provide an impurity layer with low electrical resistance that is effective as a window side impurity layer of non-quality semiconductor solar cells. <<Means for Solving the Problems>> The above-mentioned objects of the present invention are to provide a pin type solar cell having a textured transparent electrode and a non-quality semiconductor layer,
The window side impurity layer of the solar cell is an amorphous silicon layer containing microcrystals (microcrystalline M) and an amorphous silicon nitride M (
The window side impurity layer has a multilayer structure of 100%
This achievement was achieved using an amorphous semiconductor solar cell, which is characterized by a film thickness that exceeds that of humans. As the transparent electrode substrate used in the present invention, it is usually preferable to use glass, but other transparent materials such as plastics can also be used.

The surface of the substrate is usually flat, but in order to increase the light absorption efficiency, it is preferable to use a substrate with an uneven surface of about 0.01 to 3 μm. The shape of the unevenness may be hemispherical, pyramidal, or an intermediate shape thereof.

The transparent vAt electrode substrate with the texture structure used in the present invention has rTOi, SnO. It can be obtained by forming a film to make the surface uneven, by welding flat glass and fine powder glass to produce a glass substrate with an uneven surface, by partially etching a transparent conductive film, etc. can. In addition, there is also a method proposed by the present inventors of bringing a material with an uneven surface into contact with glass to transfer the unevenness to the glass surface (Japanese Patent Application No. 63-209823), and a method using organic aluminum and a diluent gas. It can also be produced by a method such as preparing a raw material gas and forming a thin film of aluminum having a textured structure on a glass substrate by plasma CVD (Japanese Patent Application No. 1-31946). IT O'P S n on the board
The method for providing the transparent electrode such as O z can be appropriately selected from known methods such as electron beam evaporation, sputtering, CVD, spray, and CMD. Since a combination of a non-quality silicon nitride layer and an amorphous silicon layer containing microcrystals is selected as the multilayer window-side impurity layer according to the present invention, a particularly good pin type solar cell element can be obtained. In this case, the amorphous silicon containing microcrystals can contain hydrogen and/or halogen atoms. As the halogen atom, fluorine and chlorine are preferred, with fluorine atom being particularly preferred. A non-quality silicon thin film containing such microcrystals can be formed by plasma CVD. As the source gas, silicon-containing compounds containing hydrogen atoms, such as silane and disilane, are used. The raw material gas is diluted with hydrogen and supplied to the plasma CvD apparatus. The amount of hydrogen used is 20 to 1,000 times the amount of raw material gas. At a magnification of 20 times or less, it is impossible to form good microcrystals, and at a magnification of 1,000 times or more, the deposition speed is too slow to be practical.

The discharge power should be low, below 0.1 W/cd. 0,
If it exceeds IW/d, the surface of the transparent electrode will be reduced and damaged by the hydrogen plasma, which is not preferable. on the other hand,
0. Since the film forming efficiency decreases below OIW/cd, it is particularly preferable to set the discharge power to 0.01 to 0.IW/cd. The substrate temperature is in the range 100-350'C, especially 1
The temperature range is preferably 50 to 250°C. If the temperature is below 100°C, the substrate temperature (1
At temperatures above 350°C, the amorphous S of the hetero p-layer formed first may deteriorate.
This is not preferable since it may cause thermal deterioration of the iN layer.

The pressure during discharge is preferably in the range of 500 to 1,500 mTorr, particularly in the range of 800 to 1.300 mTorr. Outside the above range, dark electrical conductivity decreases, which is not preferable. The main feature of the present invention is that it employs a microcrystalline layer formed under the conditions of high hydrogen dilution and low discharge power as described above. According to this method, the electrical conductivity can be reduced to 1.0% while keeping the optical absorption coefficient small. 5 X 1 0-' to 5 X 1 0-'
A microcrystalline layer having an excellent performance of Scm-' can be obtained. By applying high hydrogen dilution conditions, the long surface of the microcrystalline layer is coated with hydrogen, increasing the surface diffusion coefficient of reactive species, preventing the bonding of silicon atoms to unstable sites, and hydrogen. It is thought that by separating the silicon atoms bonded to unstable sites using radicals (etching reaction), the film can be made stronger by only the silicon atoms bonded to stable sites, and microcrystals can be created. In addition, by setting low discharge power conditions, the existence of a thin (approximately 50 or less) heterolayer can not only suppress damage caused by hydrogen plasma to the transparent electrode, but also prevent damage to the film quality due to temperature, pressure, etc. It is also possible to reduce the influence of The window-side impurity layer is particularly preferably a P layer, and dopants can be easily introduced into the non-quality silicon semiconductor by a known method. Formed on the amorphous silicon layer (transparent electrode side) containing microcrystals as described above,
The other amorphous silicon heterolayer forming the multilayer structure contains at least hydrogen atoms and nitrogen atoms.

In order to produce a silicon thin film containing hydrogen atoms and nitrogen atoms as impurities by plasma decomposition, a silicon compound containing hydrogen atoms such as silane or its derivatives is used as a raw material gas, such as NHs, N*Ha, N
oSNt A plasma atmosphere may be created by adding a gas containing nitrogen atoms such as O or NOx together with a dopant gas. No, no! When an impurity gas containing oxygen atoms is added, as in the case of adding an impurity gas containing oxygen atoms, it is possible to produce a silicon nitride thin film containing not only nitrogen atoms but also oxygen atoms as impurities. Particularly preferred is N
Hs. Furthermore, when generating plasma, hydrogen gas may be used as a carrier gas for discharging. The physical properties of non-quality silicon semiconductor thin films largely depend on the types and amounts of impurities they contain. for example,
The presence of nitrogen atoms contained in the amorphous silicon semiconductor causes the structure of silicon nitride (~5.0eV) with a large band gap to partially exist, so the presence of nitrogen atoms in the amorphous silicon (a-St) semiconductor The bandgap is widened, which allows the absorption coefficient to be reduced. Furthermore,
The same applies to the refractive index, and silicon nitride (refractive index 2.0
By partially having the structure 5), the refractive index of the a-Si semiconductor can be controlled. Similarly, the band gap of silicon oxide is 9. While it is large at OeV, the refractive index is 1.
46, it is possible to increase the transparency of the window-side impurity layer by including oxygen atoms as impurities, and to bring its refractive index close to that of the transparent electrode. In addition, since N and O can be incorporated into the four-coordinate Si network, they alleviate the structural stress of the amorphous silicon semiconductor thin film and, as a result, improve the adhesion to the metal substrate or transparent electrode substrate. It can be improved. On the other hand, hydrogen contained as an impurity in the amorphous silicon semiconductor thin film eliminates the dangling bonds of silicon atoms that occur when forming the amorphous silicon thin film, and reduces the density of localized levels in the silicon thin film. Therefore, it is significant for controlling the electrical resistance of the semiconductor device according to the present invention.

The total thickness of the multilayer window-side impurity layer in the present invention is 10
Try to exceed 0 people. If it is less than ioo, the open circuit voltage will not improve and this is not preferable. A particularly preferable thickness is 120 to 400 people.

Among these, the thickness of the p-type microcrystalline layer is preferably 60 to 340, particularly preferably 80 to 300. If it is less than 60 people or more than 340 people, it is not preferable because the open circuit voltage and conversion efficiency will decrease. On top of such a multilayer window-side impurity layer, an i-layer is formed by a known method, and then an n-layer is formed.
A pfn type or nip type solar cell element can be obtained by forming a layer or pli and bonding it to an electrode. In this case, the impurity layer on the electrode side other than the window side is an a-StsH layer with normal impurity control or a non-quality hydrogenated silicon rri (μc-Si) containing microcrystals.
:H) etc. can be used.

Methods for sequentially depositing each non-quality silicon film on a transparent electrode include plasma CVD, sputtering, thermal CVD, and optical CVD.
An appropriate method can be selected from among the known methods for manufacturing non-quality films, such as VD. That is, when using the plasma CVD method, for example, a predetermined reaction gas is introduced into a vacuum reaction tank in which a transparent substrate on which a transparent electrode is deposited is installed to achieve a predetermined internal pressure, and then high-frequency discharge and low-frequency discharge are applied. It can be manufactured by forming plasma using any method, including various discharge methods such as direct current discharge. Apart from sputtering, when CVD is used,
As mentioned above, a reaction gas is required. Among these CVD methods, it is particularly preferable to employ the plasma CVD method from the viewpoint of reaction efficiency. Next, the raw material gas for producing the solar cell of the present invention by the CVD method will be described. The silicon source that can be used in the present invention has the general formula S 1 n
Silane represented by H trot or general formula S i H
It can be arbitrarily selected from halogenated silanes represented by @-3Xa-+ (X represents a halogen atom). These may be used alone or in combination of two or more.

- The ratio of the silicon-containing compound forming the reaction gas and the raw material gas for impurities can be changed as appropriate depending on the desired physical property values and reaction conditions.

For example, the input power for plasma generation is approximately 0.02W.
/cj,, the reaction pressure is about 0. When the ITorr, i-plate temperature is set at about 250° C. and the gas flow rate is set at about 10 3 CCM, good results can be obtained by mixing the nitrogen-containing compound in an amount of 1 to 2 times the amount of the silicon-containing compound.

In the present invention, in order to impart p-type or n-type properties to the silicon thin film, it is necessary to further add a dopant gas within the above film forming conditions. The p-type dopant gas is an element of group Ⅰ of the periodic table of elements or a compound thereof, preferably a hydride, particularly boron hydride (B.H.). The n-type dopant is an element of group V of the periodic table of elements or a compound thereof, preferably a hydride, particularly phosphine (PH3) or arsine (AsH3). Although the mixing ratio of these dopant gases is not constant depending on the plasma conditions, good results can be obtained if the mixing ratio is about 0.1-1% by volume based on the silicon-containing compound. A typical example of the structure of the solar cell of the present invention is shown in FIG. In the figure, numeral 1 is a transparent substrate such as glass, 2 is a transparent electrode, 3 is a hetero PLL, 4 is a microcrystalline p layer, 5 is IWi, 6 is an n layer, and 7 is a metal electrode on the back surface. Each of these structural elements has a function similar to that of a so-called pin type element. The thickness of the i layer is 300~4,
000, but according to the present invention 6
Even if it is a thin film with a thickness of 0.00 or less, the open circuit voltage and fill factor will not decrease.

<<Effects of the Invention>> The solar cell of the present invention has a light trapping effect due to the textured structure of the upper transparent electrode with less light absorption by the window-side impurity layer, and has a higher conversion efficiency and lower open circuit voltage than conventional pin type elements. The present invention is significant because the fill factor does not decrease.

Hereinafter, the present invention will be explained in more detail with reference to Examples, but the present invention is not limited thereto.

{Example} A solar cell having the structure shown in FIG. 1 was manufactured using the plasma CVD method. First, 100 silane gas (SIH.) was placed in a reaction chamber equipped with a glass substrate (manufactured by Asahi Glass) with a haze rate of 5% and a textured transparent electrode (membrane width: 8,000).
A mixed gas of approximately 0.3% by volume of BtHa based on SCCM, 153 CCM of ammonia gas, and silane gas was added at a pressure in the reaction chamber of approximately 0.0%. It was introduced to become ITorr.
Next, plasma discharge was started with an input power of 10 W, and the transparent electrode was heated at a substrate temperature of 250°C. Approximately 20 heteropils
l(3) was deposited. Next, after evacuating the reaction chamber, silane gas and a mixed gas of 0.2% by volume of Bz Ha to the silane gas were diluted 50 times with hydrogen gas and introduced, and the pressure inside the reaction chamber was reduced to 0.
．． It was set to 3 Torr. Input power 20W, board temperature 250
℃ conditions, an amorphous silicon layer (4) containing about 130 microcrystals was formed on the heteropJi, resulting in a total of 150 window-side impurity layers. On the impurity layer formed in this manner, an i-layer and an n-layer were further formed using a conventional method, and metal electrodes were attached to obtain a sample (D) of the present invention. The i-layer and nil are exactly the same as this sample, and the window-side impurity layer is pc
Comparative sample (A) with a single layer of -St:H, a-SiN
Table 1 shows the results of comparing a comparative sample (B) with a single layer of :H and a comparative sample (C) with a flat substrate. 1st
The results in Table 2 demonstrate that the solar cell obtained by the present invention has extremely high photoelectric conversion efficiency.

[Brief explanation of drawings]

Claims (1)

[Claims] A pin-type solar cell having a transparent electrode with a textured structure and an amorphous semiconductor layer, wherein the window-side impurity layer of the solar cell includes an amorphous silicon layer (microcrystalline layer) containing microcrystals and an amorphous siliconite layer. 1. An amorphous solar cell having a multilayer structure of ride layers (heterolayers), wherein the window-side impurity layer has a film thickness exceeding 100 Å.